It is well known that, among semiconductor nanoparticles, nano-sized particles having a smaller particle size than an electron wavelength (approximately, 10 nm) are greatly affected by the finite nature of particle size on the motion of an electron, as a quantum size effect and exhibit specific physical properties differing from its bulk body. A semiconductor nanoparticle which is composed of a nanometer-sized semiconductor material and exhibits a quantum confinement effect is generally called a quantum dot. Such a quantum dot, which is a small agglomerate of some ten nms and composed of some hundreds to some thousands of semiconductor atoms, emits an energy equivalent to the energy band gap of the quantum dot when absorbing light from an exciting source and reaching an energy-excited state.
Accordingly, it is known that a quantum dot exhibits unique optical characteristics through a quantum size effect. Specifically, a quantum dot exhibits characteristics such that (1) control of particle size renders it feasible to emit light of various wavelengths or colors, (2) it is possible to allow particles of various sizes and exhibiting a broad absorption band to emit light by the exciting light of a single wavelength, (3) a fluorescence spectrum exhibiting a favorable symmetrical form, and (4) it exhibits enhanced durability and excellent fade resistance, compared to organic dyes.
On the other hand, there has been noted in vivo light imaging for small animals and optical system devices which are capable of externally observing cells within the living body of a small animal without injuring the living body (non-operatively) have been commercially available from various manufacturers. This is a method in which a labeled fluorescent material capable of selectively gathering at a site to be observed within a living body is injected into the living body and is externally exposed to exciting light and the emitted light is externally monitored. There has been studied a technique of employing, as a means for labeling a biomaterial, a biomaterial labeling agent of a molecular labeling substance being attached to a marker substance.
Recently, there has been noted a technique of employing a quantum dot as the foregoing marker substance. For instance, there has been studied a biomaterial labeling agent in which a polymer having a polar functional group is physically and/or chemically attached to the surface of a quantum dot (as described in, for example, patent document 1).
Recently, there have been desired biomaterial labeling agents exhibiting enhanced emission intensity to perform high-sensitive biomaterial detection. As one of them was disclosed a method of allowing plural quantum dots to be embedded within a silica nanoparticle (as described in, for example, patent documents 2 and 3).
However, it was proved that, when a quantum dot-embedded silica was prepared in accordance with the method disclosed in the patent document 2 by the inventors of the present invention and its emission intensity was measured, the emission intensity was lower than that expected from the number of quantum dots, requiring an additional improvement.
Further, the method disclosed in the patent document 3 employed a nonionic surfactant in the preparation process. Such a nonionic surfactant possibly causes denaturation of a biomaterial and in cases when using a quantum dot-embedded silica, it was necessary to require a step for removing such a surfactant, producing industrial problems.